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# Help for calculating speed of and DC motor.

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#### Greatbuddy

##### New Member
Hello everyone,
I required a circuit for calculating the speed of a DC motor. Is it possible to design a one like that? Please give your ideas and suggestions for this. I will attach datasheet of my DC motor.

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• DC motor.pdf
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#### Grossel

##### Well-Known Member
What do you "have" in hands? You need to provide more information on pretty much everything.

Also you can try to search for "DC motor control" on the web.

To basically control the speed of a DC motor you typically need a RPM to voltage converter (you can make one by disassemble an old computer mouse with ball inside and the circuitry) and some circuit to vary the speed. The latter often consists of a PID controller and a switching voltage supply.

#### Greatbuddy

##### New Member
I need an idea and in hand i have nothing. We are going to procure a dc motor. Since it costs much. I havent asked about "DC motor control". I need an idea from you guys.

#### twinklestar

##### New Member
sorry buddy i have no idea........

#### Chippie

##### Member
Hello everyone,
I required a circuit for calculating the speed of a DC motor.

You need a tachometer circuit...

You need to generate a pulse or number of pulses for every turn of the motor spindle, then count the number of pulses generated over a fixed time period...then convert that to revolutions per minute...

Do a search for Tacho or Tachometer...

#### MrAl

##### Well-Known Member
Hi,

Many dc motors can also be controlled using a purely analog circuit.
They work on the principle that the back emf voltage is related to
the speed of the shaft. The idea then is to create a circuit that
measures the back emf while the motor is running and then generate
a control signal based on the back emf and a reference signal.
Many older cassette tape decks used this kind of controller.

If you only need to 'measure' the speed then you can make a
circuit to measure the back emf and that will give you the
scaled speed measurement.

Last edited:

#### Bob Scott

If you don't want to or have the time to make a tach circuit, buy a model airplane tach for 2 bladed prop from a radio control hobby shop. You just need to attach a B&W drawing (4 quadrants, alternating black and white, B_W_B_W) on the shaft to measure the RPM. In 1990 I paid about $40 for my tach. Last edited: #### bailey45 ##### New Member The LM2917 will convert pulses into a voltage. An Optek OPB830L11 will create pulses when a disc with radial slots is turned inside the sensor. LM2905$2.57 Digikey, OPB830L11 Approx \$3.00

These two parts combined will give you a circuit which generates a voltage directly proportional to the speed of the motor. Check out the data Sheets

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